The invention relates to methods and apparatus, both devices and systems, for control of NOx in catalytic combustion systems, and more particularly to control of thermal or/and prompt NOx produced during combustion of liquid or gaseous fuels in the combustor sections of catalytic combustor-type gas turbines, by controlled injection of water in liquid or vapor form at selected locations, orientations, amounts, rates, temperatures, phases, forms and manners in the combustor and/or compressor sections of gas turbines. The ratio of NOx ppm reduction to water addition, in weight %, is on the order of 4-20, with % NOx reduction on the order of up to about 50-80%, or more, and NOx of below 2 ppm being achievable by the inventive process.
Gas turbines are used for a variety of purposes, among them being motive power, gas compression and generation of electricity. The use of gas turbines for electrical generation is of particular growing interest due to a number of factors, among them being modularity of design, good ratio of generation output capacity to size and weight, portability, scalability, and efficiency. They also generally use low sulfur hydrocarbon fuels, principally natural gas, which offers the promise of lower sulfur oxides (SOX) pollutant output. This is particularly important in the case of use of gas turbines for power generation in urban areas, where they are attractive for grid in-fill to cover growing power needs as urban densification occurs.
However, gas turbines operate at high temperature, in the range of from about 1100xc2x0 C. for moderate efficiency turbines, to 1500xc2x0 C. for modern high efficiency engines. To achieve these temperatures at the turbine inlet, the upstream combustor section must produce a somewhat higher temperature, generally 1200 to 1600xc2x0 C. to compensate for air infiltration as a result of seal leakage or the purposeful addition of air for cooling of the metal walls. At these temperatures, the combustion system will produce NOx, in amounts increasing as the temperature increases. The increased amounts of NOx need to be reduced to meet increasingly stringent emissions requirements.
Current Gas Turbine Systems
A typical gas turbine system comprises a compressor upstream of, and feeding compressed air to, a combustor section in which fuel is injected and burned to provide hot gases to the drive turbine which is located just downstream of the combustor section. FIG. 1 shows a conventional system of the type described in U.S. Pat. No. 5,183,401 by Dalla Betta et al., U.S. Pat. No. 5,232,357 by Dalla Betta et al., U.S. Pat. No. 5,250,489 by Dalla Betta et al., U.S. Pat. No. 5,281,128 by Dalla Betta et al., and U.S. Pat. No. 5,425,632 by Tsurumi et al. These types of turbines employ an integrated catalytic combustion system in the combustor section. Note the combustor section comprises the apparatus system between the compressor and the drive turbine.
As shown in FIG. 1 the illustrative combustor section comprises: a housing in which is disposed a preburner; fuel source inlets; catalyst fuel injector and mixer; one or more catalyst sections; and a post catalyst reaction zone. The preburner burns a portion of the total fuel to raise the temperature of the gas mixture entering the catalyst, and some NOx is formed there. Additional fuel is introduced downstream of the preburner and upstream of the catalyst and is mixed with the process air by an injector mixer to provide a fuel/air mixture (F/A mixture). The F/A mixture is introduced into the catalyst where a portion of the F/A mixture is oxidized by the catalyst, further raising the temperature. This partially combusted F/A mixture then flows into the post catalyst reaction zone wherein auto-ignition takes place a spaced distance downstream of the outlet end of the catalyst module. The remaining unburned F/A mixture combusts in what is called the homogeneous combustion (HC) zone (within the post catalyst reaction zone), raising the process gases to the temperature required to efficiently operate the turbine. Note that in this catalytic combustion technology, only a portion of the fuel is combusted within the catalyst module and a significant portion of the fuel is combusted downstream of the catalyst in the HC zone.
Each model and type of drive turbine has a required inlet temperature, called the design temperature or turbine inlet temperature. In addition, because cooling air is injected just upstream of the drive turbine, the outlet temperature of the combustor must in fact be higher then the turbine inlet temperature. For proper operation of a gas turbine at high efficiency, the combustor section outlet temperature must be continuously controlled to be maintained at the desired combustor outlet temperature. Typically, the turbine inlet temperature ranges from about 900xc2x0 C. to about 1250xc2x0 C. and the required combustor outlet temperature can be as high as 1500xc2x0 C. to 1600xc2x0 C. At these high temperatures, additional NOx is formed in the post catalyst reaction zone of the combustor section of FIG. 1. Although the NOx level produced in the catalytic combustor is typically low for natural gas and similar fuels, it is still desirable to reduce this level even further to meet increasingly stringent emissions requirements.
The relationship between temperature in the turbine combustor section and NOx produced therein is shown in FIG. 2. FIG. 2 shows the level of NOx that ordinarily is produced in a combustor of the type shown in FIG. 1. At temperatures below about 1450xc2x0 C., identified in the figure as Region A, the level of NOx produced is below 1 ppm. As seen in FIG. 2, at temperatures above about 1450xc2x0 C., the Region B lower boundary, the NOx level rises rapidly, with 5 ppm produced at 1550xc2x0 C., and even higher levels, 9 to 10 or more ppm, above that temperature. For gas turbines that require combustor outlet temperatures in Region B to achieve the drive turbine design (inlet) temperatures, and where emissions requirements demand emissions levels below 2 ppm, it becomes necessary to further modify the combustion system, including combustion process, apparatus and controls, to maintain the NOx level produced in the combustion section of a gas turbine system at lower NOx levels, for example, 2 ppm or less.
The top portion of FIG. 3 is an enlarged schematic of a portion of FIG. 1 showing the major components of a catalytic combustion system 12 located downstream of the preburner. The cataltic combustion system includes a catalyst fuel injector 11, one or more catalyst sections 13 and the post catalyst reaction zone 14 in which is located the HC (homogeneous combustion) zone 15. The bottom portion of FIG. 3 illustrates the temperature profile and fuel composition of the combustion gases as they flow through the combustor section described above. Temperature profile 17 shows gas temperature rise through the catalyst as a portion of the fuel is combusted. After a delay, called the ignition delay time 16, the remaining fuel reacts to give the full temperature rise. In addition, the corresponding drop in the concentration of the fuel 18 along the same path is shown as a dotted line.
Water Addition in Non-Catalytic Systems
A. Bhargava, et. al, in ASME 99-GT-8, 1999 reports on the addition of water to a fuel-air mixture, combusting it in a flame combustor and measuring the NOx level. That work was not done on a catalytic combustion system, but rather was done in a premixed combustion system in which the fuel and air is premixed prior to combustion. Further, the system tested by Bhargava, et. al. was a flame combustor system, as compared to a flameless catalytic system. The Bhargava combustion process relies on recirculation and other mechanisms to stabilize the flame combustion process. The Bhargava et al. report does show that water addition to the premixed fuel air mixture introduced into a flame combustor does reduce the NOx level produced by the flame combustor. Flame combustors may be used upstream of catalytic combustion modules.
In other work, water and steam have been added to and mixed with fuel in gas turbine and other non-catalytic, flame-type combustors for the purpose of reducing NOx. See: G. Touchton, ASME, 84-JPGC-GT-3, 1985; F. Dryer, Sixteenth International Symposium on Combustion, The Combustion Institute, p. 279, 1976; T. Miyauchi et. al. Eighth International Symposium on Combustion, The Combustion Institute, p. 43, 1981; J. Meyer and G. Grienche, ASME, 97-GT-506, 1997; L. Blevens and R. Roby, ASME, 95-GT-327, 1995. The process by which such NOx reduction occurs is through the reduction of the temperature in the combustor flame zone. The fuel is mixed with water or steam, and this fuel/water mixture is then injected into the combustor and burned in a typical diffusion flame. The added mass of water, in either steam or liquid form, reduces the hot spot temperature of this type of flame combustor, thereby reducing the NOx level. The temperature reduction is due to the high heat capacity of the water or steam, and in the case of liquid water, additionally has a high heat of vaporization. The effect of the water or steam addition is to reduce the flame hot spot temperature so less NOx is produced at the lower temperatures. None of Bhargava, Touchton, Miyauchi, Meyer or Blevens disclose employing controllers for continuous control of NOx by control of water addition.
Catalytic Combustion Systems Are Different in Kind
However, flameless catalytic combustion systems are not the same as flame combustors, as is evident from FIG. 3, as a result of which the introduction of water into a catalytic system is not predictable. Indeed, catalytic combustion systems do not have localized high temperature spots nor do they employ recirculation, so addition of water for hot spot control is not needed. Further, water addition would be expected to quench the catalyst, or reduce the temperature being produced at the outlet of the combustor section to below the required drive turbine design temperature. Accordingly, one of ordinary skill in this art would not consider water addition to catalytic combustion systems, nor would they expect that water addition to a catalytic system would lead to reduction in NOx. 
Thus, there remains a significant need for NOx control and reduction in gas turbines, and more specifically in gas turbine combustor sections employing catalytic combustion systems.
The invention is directed to methods and apparatus, both devices and systems, for control of NOx in catalytic combustion systems, and more particularly to control of NOx produced during combustion of liquid or gaseous fuels in the combustor sections of gas turbines by controlled injection of water in liquid or vapor form at selected locations, orientations, amounts, rates and manners in the combustor and/or compressor sections of gas turbines.
The invention arises out of the discovery that the addition of water into at least one of a compressor and a combustor section having a catalytic combustion system has the effect of reducing the NOx produced in the post-catalyst homogeneous combustion zone downstream of the catalyst. The water may be introduced in a wide variety of modes, locations, amounts, rates, temperatures, phases and orientations, both in the combustor section and upstream of it in the compressor. For example, it has been found that the addition of water to the gas mixture that is fed to the catalyst module will reduce the NOx produced in the post catalyst homogeneous combustion zone by up to approximately 80% or more, to a concentration below about 2ppm NOx in the hot gases being introduced into the turbine.
By xe2x80x9cadditionxe2x80x9d or xe2x80x9cintroduction of waterxe2x80x9d is meant addition or introduction of water in any phase or temperature, e.g., stream, spray, atomized or vapor form, the latter including hot water vapors or steam. The water can be hot, ambient, cool or cold, typically ranging from about xe2x88x9210xc2x0 C. to over 400xc2x0 C.
It should be understood that by reference to xe2x80x9cairxe2x80x9d is meant broadly the process gases flowing through the entire turbine system, as they change from air at the compressor inlet to combustion gases of varying oxygen content in the combustor section to the ultimate xe2x80x9cproductxe2x80x9d hot gases stream at the discharge end of the post catalyst combustion zone of the combustor section.
It should be understood that a catalytic combustion system, properly operated, does not have a localized high temperature zone. Thus, the added water, considered as a diluent, needs to be compensated-for by added fuel to maintain the combustor outlet temperature. In the case of a catalytic combustion system such as that shown in FIG. 1, the overall combustor outlet temperature must be maintained at the level required by the gas turbine, i.e., the design temperature or turbine inlet temperature. Compensating amounts of fuel added to the preburner and injector/mixers is typically controlled in a straight-forward manner by the turbine control system.
It should be understood that the addition of water also can be strategically employed at various locations in selected amounts within the compressor and combustor sections to control the temperature profile shown in the bottom of FIG. 3. In this connection, it should be understood that it is within the scope and teachings of the inventive process to introduce water simultaneously in a plurality of locations in the compressor and combustor sections, in differing amounts, and/or at different rates, and/or at different temperatures or forms, to obtain the desired temperature profile and combustor outlet temperature control. Likewise, the inventive process includes introduction of water in programmed sequences at different locations in different amounts, at different rates, temperatures, phases, forms and modes in those sections. Such simultaneous and/or sequential introduction at multiple points and in varying amounts/rates/etc., along the gases path through the combustor and/or compressor section(s) may also be changed depending on the turbine operating cycle, from start up, through spool up, at load, during turn down, and shut off, and during changes in load cycle. The inventive process includes introduction of the water in accord with a suitable control algorithm, such as monitoring the temperature at one or more locations in the combustor and/or compressor section(s), monitoring the NOx in the post catalyst reaction zone or elsewhere along the process air path, and controlling the amount, location, temperature, form (e.g., phase or droplet size), mode and rate of water fed into the process to maintain the outlet temperature of the gases at the required temperature for efficient turbine operation.
The addition of water (in all cases xe2x80x9cwaterxe2x80x9d refers to either liquid water, steam or superheated steam) can be used to reduce NOx in combustion systems operating at reaction zone temperatures above 900xc2x0 C. It is preferably applied to combustion systems operating at combustor outlet temperatures of 1400xc2x0 C. to 1700xc2x0 C., and most preferably at combustor outlet temperatures of 1450xc2x0 C. to 1600xc2x0 C. The amount of water added is sufficient to provide a concentration of water in the range of from about 0.1% to about 20% by weight of the total air and fuel mixture flowing into the post catalyst reaction zone. The preferred amount of water addition is in the range of about 0.5% to about 10%, and the most preferred range of water addition is from about 0.1% to about 5% by weight. As the amount of water added is increased, the effect on the NOx will be dependant on a number of factors including the reaction zone temperature range and the residence time at the reaction zone temperature. The ratio of NOx reduction in ppm to water addition in weight % ranges from on the order of about 4 to about 20.
The purity of water is also an important consideration, and it is a principle of the invention that good to high quality water is preferably employed so as to not introduce additional corrosive or catalyst poisoning components, or flame retardants or pollutant-causing or contributing components, molecules or ions. It is particularly important to use high purity water when injecting water upstream of the catalyst module, as the catalyst can be poisoned by a variety of water-born compounds. In addition, contaminants in the water can adversely impact the durability of the turbine downstream of the combustor.
For typical systems with reaction zone temperatures of from about 1400xc2x0 C. to about 1600xc2x0 C. and residence times of 3 to 20 ms (milliseconds), the addition of 1% weight water to the weight of total mass flow through the combustor will reduce the NOx by about 15%, at 2.5% wt/wt water the NOx will be reduced by about 30%, and at about 5% wt/wt water the NOx reduction is about 50% from the levels achievable without the addition of water.
By way of additional embodiments and advantages, the inventive process and apparatus for control of NOx via introduction of water include the following alternative water addition modes and/or locations, and control systems for the additions:
Water (as defined herein) is introduced at the compressor inlet or inter-stage in the compressor, where it can do double duty, both NOx reduction and providing an inter-cooling effect, particularly where the water is introduced as liquid water and is ambient, cool or cold. This can result in increased turbine power (for water introduction at the compressor inlet) and/or efficiency (both power and efficiency are increased by water addition interstage in the compressor).
Water can be introduced at the compressor discharge area, or in any location upstream of the preburner, if there is one. In this case the water will reduce the NOx produced in the preburner. In addition, the long passage of the air and water to the catalyst will help to mix the air and added water, and the catalyst fuel mixer system will further mix the air with the added water.
The water can be added by intermixing with the fuel, e.g., in the Fuel/Air injector/mixer (xe2x80x9cCatalyst fuel injector and mixerxe2x80x9d) for the catalyst fuel as shown in FIG. 1, so the mixer does double duty, mixing the fuel and air, and mixing the fuel/air mixture with the water.
The fuel for the catalyst module is injected through a fuel peg comprising a cylindrical pipe having an internal passage for fuel and holes through the pipe wall directing the fuel into the air flow stream. In this embodiment of the invention, in a first alternative system and method, the water is added in a plurality of adjacent pegs specifically designed for water injection. In a second alternative of this embodiment, the water is injected via multiple passages within the fuel injection peg. In the latter embodiment, the water and fuel passages or lines to the fuel peg (spray heads or stubs) or injectors can be separate lines, or common lines can be sized and materials selected to be compatible with the mixture of fuel and water.
The water may be added downstream of the catalyst module, e.g., at or adjacent to the catalyst module outlet. In this embodiment, the water is piped to one or more distribution spray manifolds via the centerbody of the module, i.e., the module central axial rod or spindle that can be hollow to carry the water. Alternatively, the water can be introduced through the periphery of the liner downstream of the catalyst. The Fuel/Air ratio can be adjusted, and the fuel supplied can be increased to maintain the combustor outlet temperature. In this embodiment, the water purity requirement can be reduced to that required by the turbine, rather than a possibly higher purity required for water injection upstream of the catalyst module to guard against catalyst contamination.
Water (preferably steam) introduced at the catalyst module outlet is combined with additional fuel, and this fuel is burned in the post catalyst reaction zone, primarily in the HC zone, to obtain even higher temperatures at the combustor outlet. The fuel and steam, introduced separately but preferably as a mixture, will have greater mass, and will be more readily mixed into the hot process gases exiting the catalyst.
In another embodiment, the water can be injected upstream of the preburner to reduce preburner NOx. In a typical catalytic combustion system used in a combustor section, the preburner is a premix preburner. In contrast, in typical combustors not employing catalytic combustion, a diffusion flame preburner is typically employed. The diffusion flame preburners are prone to producing NOx. In accord with the process of this invention, water can be injected in association with a diffusion flame preburner. The advantages are that the diffusion flame preburners with water injection will then produce low NOx, and the water will then flow to the catalytic combustion system where it will then further reduce the amount of additional NOx produced in the post catalyst reaction zone of the catalytic combustion system. Water injection is also useful for reducing NOx in a lean premix preburner.
Water can be injected between stages of a multi-stage combustion catalyst module, with water being conveniently ported to the appropriate stage via the centerbody or via the periphery of the catalyst module.
Waste heat in the gas turbine exhaust, or in the exhaust of a downstream boiler can be recovered and used to convert water into high pressure steam which is then injected into the combustor section for the NOx control in accord with the principles of the invention. This minimizes energy consumption to evaporate and heat the water with a resulting increase in overall process efficiency. Thus, the water introduction system of the invention also provides a vehicle for efficient recovery and feed-back of heat into the combustion portion of the turbine system.
Catalyst modules may contain both catalyst-containing and catalyst-free (non-catalytic) channels. In such type of catalyst modules, the water can be introduced through the non-catalytic channels, with the advantage of eliminating effects of water on the catalyst coating. This also reduces the fuel content in the non-catalytic channels, which has the advantage of reducing the potential for hydrocarbon build up on the channel walls (soot or char build up) or of combustion of the fuel in these channels.
It should be understood that the embodiments described herein are exemplary only of the principles of the invention, and more than one mode and location of water introduction may be used simultaneously or sequentially. Further, different modes and locations of water introduction may be used at different times in the cycle of start-up, spool-up, continuous operation (including turn-up or turn-down to meet output load demand), and shut-down. The purity, amount, rate of introduction, temperature and form or phase, and liquid droplet size of water introduced in each location may be controlled empirically through feed-back controllers measuring, inter alia, temperature, NOx production, fuel usage and the like, or in accord with control system models or algorithms, including conventional, commercially available controllers, particularly where the water is premixed in the fuel.
A wide variety of control systems and controllers may be used, e.g, feed-back control system controllers, dynamic control systems, operating line or operating chart control systems, open loop or closed loop systems, and feed-forward control systems, to monitor operation of the turbine, the compressor section and the combustor section for appropriate water addition in accord in the principles of the invention. A control strategy and control system that is particularly useful by which water can be introduced into the combustor and/or compressor section in accord with this invention is shown in our copending Application U.S. Ser. No. 09/942,976, of Yee, Velasco, Nickolas and Dalla Betta, filed Aug. 29, 2001, entitled CONTROL STRATEGY FOR FLEXIBLE CATALYTIC COMBUSTION SYSTEM, the disclosure of which is hereby incorporated by reference to the extent required to enable one skilled in the art to adapt such controllers to include introduction of water in accord with the inventive process.
Other examples of control systems and fuel injection systems by which water may be introduced into the combustor and/or compressor section in accord with this invention include the following: Reed et al., U.S. Pat. No. 4,283,634 (1981xe2x80x94Westinghouse), describing a system and method for monitoring and controlling operation of industrial gas turbine apparatus and gas turbine electric power plants preferably with a digital computer control system; Tyler, U.S. Pat. No. 5,095,221 (1992xe2x80x94Westinghouse), disclosing a gas turbine control system having partial hood control; and Kiscaden et al., U.S. Pat. No. 4,380,146 (1983xe2x80x94Westinghouse) disclosing a system and method for accelerating and sequencing industrial gas turbines and electric power plant gas turbines by a digital computer control system.